The present invention relates to powder material compacting presses, more particularly to an improved die and punch assembly for powder compacting presses and to the method practiced thereby.
In powder compacting presses as disclosed in U.S. Pat. Nos. 3,328,840, 3,344,213, 3,328,842, 3,414,940, 3,561,054, 3,726,622, 3,741,697, 3,775,032, 3,805,370 and 3,822,974, all assigned to the same assignee as the present application, there are disclosed apparatus such as presses and tools for such presses for compacting powder material, such as powdered metal, ferrite, glass and other materials into diverse articles such as toroids, beads, pellets and the like. In the powder compacting apparatus disclosed in the aforementioned patents, the articles are formed in single or multi-cavity dies, in which reciprocable punches are disposed, by compaction of the powder material between the punch end face and an anvil displaceable over the die cavity so as to overlap the die cavity.
A work station positioner assembly, forming part of the press apparatus, is disposed angularly or linearly movable over the die plate and is provided with three separate or integral elements, a powder dispenser unit, an anvil, and a pick-up head. The powder dispenser unit is first positioned over the die cavity to fill the die cavity with a predetermined amount of powder material. The dispenser unit is then removed from above the die cavity, and the anvil unit is in turn positioned over the die cavity and clamped in position. The punch is reciprocated upwardly in the die such as to compact the powder material between the punch end face and the anvil. The anvil is then unclamped from above the die cavity and replaced by the pick-up head as a result of further angular or linear motion of the work station positioner assembly. The punch is reciprocated upwardly so as to eject the compacted article from the die cavity into the pick-up head for transfer to a remote station, or, alternatively, for transfer to a collection station by subsequent motion of the work station positioner assembly.
By way of utilizing standardized punch and die assemblies in the form of interchangeable tool capsules, all adapted to be interchangeably mounted on the press table in an appropriate mounting aperture and held therein by any convenient means such as by mounting bolts or clamps, the remaining of the tool capsule projecting below the press table, with the punch actuating mechanism of the press appropriately connected to the punch actuating plate portion of the tool capsule, it is a simple matter after a production run of a particular part to remove a tool capsule and replace it by another tool capsule for compacting a different part. With the exception of the die, the punches, and the core rods, if any, all the other mechanical parts forming the tool capsule are subject to little or no wear. The die, the punch and the core rods, if any, are however, subject to important load stresses and to wear, as a result of which they may experience dimensional changes, such as a progressive opening of tolerances, and, if subjected to abnormal loads, they may be damaged beyond repair or even break. It is therefore convenient for the user to provide a tool capsule which can be easily dismantled and which provides easy removal of the die plates or of the die bushings, the punches and the core rods, when they become worn or when they break, for replacement by new die plates or die bushings, punches and core rods.
A problem often associated with the production of, for example, relatively thin and wide articles compacted of powder material, is that of obtaining constant density throughout the width of the compacted article. If the density is not constant throughout the width of the article, during "firing" or sintering of the article in a furnace and in the course of cooling after sintering, the internal stresses caused by shrinkage of the article may cause the article to warp or, as it is commonly referred to, to become "banana-shaped". Such warpage, which is directly caused by the uneven density of the compacted powder particles, results from uneven filling of the die cavity by the powder dispenser unit. The powder dispenser unit is generally in the form of a dome or bell-shaped structure, forming part of the work station positioner assembly, which is supplied in powder material from a primary powder hopper through a flexible conduit. The edge, or lip, of the dome-shaped hollow structure, forming the powder dispenser unit, is constantly in engagement with the surface of the die plate, and the interior of the powder dispenser unit is constantly filled with powder material. When placed over the die cavity, with the punch retracted to the "fill" position, some amount of powder falls by gravity in the die cavity on the top of the punch face, and when the powder dispenser unit is displaced away from the die cavity, the trailing edge or lip of the powder dispenser unit wipes the top of the powder material in the die cavity on a level desirably flush with the die plate surface, such that a predetermined mass of powder material remains in the die cavity, with its top level ideally evenly disposed in the same plane as the plane of the die plate surface. However, the friction between the trailing edge or lip of the powder dispenser unit and the powder particles in the die cavity in the course of the travel of the powder dispenser unit across the top of the die cavity tends to displace the powder particles in the die cavity in the direction of travel of the powder dispenser unit, with the result that the leading side of the die cavity tends to become underfilled while the trailing side of the die cavity tends to overfill, and the wiping action of the trailing edge or lip of the powder dispenser unit tends to slightly prepack the powder particles in the direction of travel of the powder dispenser unit, and more so at the trailing side of the die cavity. The result is that, when the anvil is placed over the die cavity, and further frictional engagement may have taken place between the face of the anvil linearly moving in the same direction as the powder dispenser unit, further migration of powder particles may take place from the leading side of the die cavity to the trailing side with additional concentration of powder particles towards the trailing side. During compaction of the article through the subsequent upward motion of the punch face towards the anvil face, resulting in compacting the article within, for example, substantially parallel faces, the density of the compacted article is greater at the trailing side of the die cavity than it is at the leading side.
It has been discovered that uneven filling of the die cavity with powder material may be compensated for by slightly inclining the face of the punch towards the leading side of the die cavity. Although the punch could be made with a face disposed at a slight angle, such a structure is not convenient as the uneven filling of the die cavity varies in accordance with a plurality of inter-reacting and unpredictable variable factors such as atmospheric moisture, slight variations in the quantity or quality of the binder mixed with the powder material, variations in the size of the powder particles, etc.